If you are a US Citizen or Permanent Resident, Border Patrol cannot prevent you from entering the United States. They can, however, detain you for up to 72 hours and confiscate the locked device if they have "reasonable suspicion". The confiscated property will be returned eventually.
If you are not a US citizen, refusal to unlock a phone and allow inspection, inclusive of allowing access to social media and corporate apps, will probably result in denied entry. They also have the right to detain you until indefinitely until you unlock the phone if they have "reasonable suspicion", but requires a court order within 72 hours.
Most foreign counties have similar rules in place for residents and non-residents.
They don't usually return the devices they steal, and most people travel with a total device value lower than the cost of an attorney and lawsuit to force the return.
https://www.cbp.gov/sites/default/files/documents/inspection...
If you are not a US citizen, refusal to unlock a phone and allow inspection, inclusive of allowing access to social media and corporate apps, will probably result in denied entry. They also have the right to detain you until indefinitely until you unlock the phone if they have "reasonable suspicion", but requires a court order within 72 hours.
Most foreign counties have similar rules in place for residents and non-residents.